Abstract 12

Effect of screw insertion torque level on cortical bone pullout strength

Tammy M. Cleek, Trevor C. Hearn, Karen J. Reynolds

Flinders University of South Australia

During insertion, screw torque can reach levels at which bone failure and stripping occurs. Surgeons reportedly induce torque levels surprisingly close to this limit, tightening to an average of 86% of failure torque. Although high screw forces are desirable for stability, a tradeoff exists between torque and screw force levels, cumulative damage from tightening, and subsequent reduction in holding strength of surrounding bone. The objective of this study was to determine the effect of insertion torque level on bone material properties as assessed by screw pullout strength. Ten pairs of ovine tibiae were tested. One side of each pair was randomly selected for measuring ultimate failure torque (Tmax) at 3-4 shaft sites along the flat medial aspect. Tmax values were normalized by bone density (measured by DXA) and used to predict Tmax at contralateral tibia sites. Screws were inserted at 50%, 70% and 90% of predicted Tmax; pullout tests were immediately performed and strength values normalized by cortical thickness. Results showed that 90%Tmax could be beyond the yield point and reduce pullout strength. Limitations include small sample size and inexact estimation of Tmax. However, one single test at 90%Tmax indicating a 50% loss in stiffness (estimated by torque-rotation data) resulted in a pullout strength value 40% less than the average 70%Tmax pullout strength. Analysis of failure tests indicate tightening to 86%Tmax leads to an average 50% loss in stiffness. We conclude that screw tightening to clinical levels may lead to damage sufficient to compromise holding strength in surrounding cortical bone.

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